Deep brain stimulation (DBS) is a surgical treatment involving the implantation of a medical device, which sends electrical pulses to specific parts of the brain. A preferred DBS probe comprises a plurality of electrodes for providing stimulating electrical pulses at different positions in the target region. For example, the probe may comprise an array of 64 or 128 electrodes. DBS in selected brain regions has provided remarkable therapeutic benefits for otherwise treatment-resistant movement and affective disorders such as chronic pain, Parkinson's disease, tremor and dystonia. DBS surgery aims to electrically stimulate a target structure, while minimizing detrimental side-effects caused by stimulation of particular nearby neuronal structures. To make that possible, it is important to know the effect of particular stimulation settings on the electrical field that is generated in the brain tissue. Likewise, it is desirable to know what stimulation settings to apply in order to obtain an optimal stimulation volume.
In ‘Electric field and stimulating influence generated by deep brain stimulation of the subthalamic nucleus’ by McIntyre et al. (2004b), Clin Neurophys 115, 589-595, a method is disclosed for developing a quantitative understanding of the volume of axonal tissue directly activated by DBS of the subthalamic nucleus. The method uses finite element computer models (FEM) to address the effects of DBS in a medium with tissue conductivity properties derived from human diffusion tensor magnetic resonance data (MRI/DTI).
It is a disadvantage of the method of McIntyre et al. that an MRI/DTI system is needed for obtaining a conductivity map of the patient's brain. Additionally, DTI does not measure electrical conductivity directly but instead estimates one by assuming a theoretical relationship between water diffusion (measured by DTI) and electrical conductivity. Furthermore, the resolution of DTI for practical scanning times is limited to about 2 mm, i.e. 4 times the typical electrode pitch of high resolution DBS probes. It is also a problem that tissue conductivity changes over time, e.g. due to encapsulation of the probe, and that the known method requires a regular update of the conductivity map. Performing regular DTI scans is unpractical for that purpose.